It is well known that ultraviolet light (UV) may be emitted by mercury/inert gas based plasmas in quartz tubes and that the wavelengths of such emissions is generally between 180 and 420 nm. Applications of such UV light include the disinfection of bacteria, very often because of the UV disturbance of DNA. Another potential application of UV light involves the photopolymerisation of liquids such as inks, adhesives and resins.
Energisation of mercury-based plasmas may be achieved by the striking of an arc across electrodes within the plasma or by the excitation of the plasma by radio frequency (rf) radiation, often at microwave frequencies around 2.45 GHz.
Energisation of UV emitting plasmas by microwave irradiation is disclosed, for example, in U.S. Pat. No. 3,872,349 (Spero et al). The advantages of microwave-energised UV emission plasmas over arc-energised plasmas are significant and include the minimisation of degradation over time and the ability to pulse and re-strike plasmas instantly giving many operational advantages in practical situations.
U.S. Pat. No. 3,911,318 (Spero et al.) discloses a microwave energised UV emission system in which ultraviolet light is emitted over 360 degrees but the light output of this technology and that of other derived and similar patents such as U.S. Pat. No. 3,943,403 (Osbourne et al.) is restricted by the attenuation of the conductive mesh that is necessary to direct and contain the microwave radiation in order to efficiently and safely energise the UV emission plasma source.
U.S. Pat. No. 6,348,669 (Little et al.) and U.S. Pat. No. 6,507,030 (Briggs et al) disclose the behaviour of an energised mercury/inert gas plasma in the form of a lossy conductor. This allows the plasma to be used to transmit rf/microwave energy if used as part of a transmission system or to attenuate it if used to block rf/microwave emission or transmission. Such methods have a disadvantage in that in a situation when a bulb is used to block the passage of microwaves, the large dimensions of the bulb and the inability of UV to be passed through an energised plasma make the efficient focussing of such light difficult and in some cases impossible. Many UV process require the high intensity that can only be achieved by focussing of emitted light. (For example, see U.S. Pat. No. 6,118,130 (Barry))
In addition, U.S. patent application Ser. No. 11/228,632 (Little and Briggs) discloses the use of tubes that are of a diameter less than 0.5 wavelength of the microwave frequency chosen (typically 2.45 GHz), to block microwave emission from a cavity provided that the length of the tube is beyond cut-off for that wavelength.